Apparatus and method for electrolytic recovery of metals

ABSTRACT

WAYS AND MEANS FOR THE ELECTROWINNING OF METAL FROM ORE IN WHICH A SLURRY OF CRUSHED ORE IS SUITABLE ELECTROLYTE IS CONTAINED INAN ELECTROLYTIC CELL THE OUTER WALL WALL OF WHICH DEFINES A REGULAR POLYGON AND IS LINED WITH SUIABLE ANODE MATERIAL. A CATHODE IS FORMED FROM INDIVIDUAL SRIPS OF INSOLUBLE METAL AND IS PREFERABLY ARANGED TO DEFINE A POLYGON HAVING THE SAME NUMBER OF SIDES AS THE ANODE LINING OF THE CELL ITSELF. THE CATHODE IS ROTATABLY MOUNTED INSIDE THE CELL AND DRIVE MEANS ARE PROVIDED TO OSCILLATE IT THROUGH A SUBSTANTIAL ARC RELATIVE TO THE FIXED ANODE. MEANS WHICH MAY BE ATTACHED TO THE CATHODE ARE ALSO PROVIDED TO AGITATE THE SLURRY TO KEEP THE ORE IN SUSPENSION AND AT THE SAME TIME CONTINUOUSLY SWEEP THE ELECTRODES CLEAN AND FREE FROM POLARIZATION FOR INCREASED EFFICIENCY A SECOND POLYGONAL SHAPED ANODE   IS PROVIDED IN THE CENTER OF THE CELL INSIDE THE CATHODE SO THAT THE CATHODE OSCILLATES OR RECIPROCATES IN A PATH LYING BETWEEN THE TWO ANODES AND EQUIDISANT THEREFROM WHEREBY BOTH SIDES OF THE CATHODE ARE FULLY UTILIZED FOR METAL DEPOSITION.

April 23, 1974 GQQLD ET AL 3,806,434

APPARATUS AND METHOD FOR ELECTROLYTIC RECOVERY OF METALS Filed Sept. 13, 1972 IF'IG.

lF/G. 2

United States Patent O 3,806,434 APPARATUS AND METHOD FOR ELECTROLYTIC RECOVERY OF METALS Reed Goold and Charles W. Wojcik, Twin Falls, and Gerald D. Cooper, Pocatello, Idaho, assignors of a fractional part interest to Wilfred H. Herrett, Filer,

Idaho Filed Sept. 13, 1973, Ser. No. 288,771 Int. Cl. C22d 1/00; C23b 5/68, 5/78 US. Cl. 204-105 R Claims ABSTRACT OF THE DISCLOSURE Ways and means for the electrowinning of metal from ore in which a slurry of crushed ore in suitable electrolyte is contained in an electrolytic cell the outer wall wall of which defines a regular polygon and is lined with suitable anode material. A cathode is formed from individual strips of insoluble metal and is preferably arranged to define a polygon having the same number of sides as the anode lining of the cell itself. The cathode is rotatably mounted inside the cell and drive means are provided to oscillate it through a substantial are relative to the fixed anode. Means which may be attached to the cathode are also provided to agitate the slurry to keep the ore in suspension and at the same time continuously sweep the electrodes clean and free from polarization. For increased efliciency a second polygonal shaped anode is provided in the center of the cell inside the cathode so that the cathode oscillates or reciprocates in a path lying between the two anodes and equidisant therefrom whereby both sides of the cathode are fully utilized for metal deposition.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to improved ways and means for the direct electrowining of metals from crushed ores. More particularly, it provides method and apparatus whereby polarization at the electrodes is inhibited and current densities maintained at high levels, well over 50 amperes per square foot of cathode area to the end that high capacity electrowinning of metals directly from crushed ores may be achieved.

Description of the prior art In electrolysis, ions in solution will, under the influence of electric current, migrate to an electrode of opposite polarity. For instance, in the case of copper oxide dissolved in sulfuric acid, the copper goes into solution as CuSO which is a known electrolyte. When current is passed through the electrolyte, the copper migrates to the cathode where it gains two electrons, becomes neutral and deposits on the cathode as pure metal. The electrolysis reaction releases hydrogen at the cathode and oxygen at the anode. The hydrogen combines with sulfate ions to generate acid which may be recycled.

Electrowinning of metals directly from crushed ores is recognized as a highly desirable goal that would enable recovery of metal while eliminating most of the steps of concentration, leaching, smelting and the like. Over the years, much work has been directed toward development of practical electrowinning systems. Although some electrowinning has been in commercial use, the systems are limited to those in which the crushed ore has, at the very least, been leached and then the leach solution subjected to electrolysis. Even these systems are not all that are to be desired. A major drawback is the fact that metal of acceptable grade has been recoverable only at relatively low current densities in the order of 8-10 amperes per square foot of cathode area. At such low current 3,806,434 Patented Apr. 23, 1974 density the rate of recovery (cell capacity) is very low thus capital costs of the plant are high.

Another disadvantage of prior electrowinning systems is the limited efficiency of extractoin even from leach solutions. For instance, in a copper circuit, a cell can usually extract no more than four grams per liter of solubilized copper from the electrolyte before undesirable oxidation sets in which reduces the current efficiency to a point where electrical costs become prohibitive. To overcome this, prior systems limit extraction to about four grams per liter then continuously purify and recycle the electrolyte to maintain high metal concentration in the cell.

Another difnculty encountered in prior work is polarization (gasification) at the electrodes which blocks metal ions. If power is increased to overcome this, the gas physically interacts with the deposited metal to form an unacceptable grainy or powder-like product.

It has been suggested that stirring the electrolyte or rotation of electrodes will overcome polarization. This has been tried with some success, but has not yielded commercially acceptable results in terms of cell capacity, current efiiciency or purity of metal.

SUMMARY OF THE INVENTION It is the primary object of this invention to provide ways and means for the direct electrowinning of metal values from crushed ore without prior leaching or separation of electrolyte from leached ore.

Another object is to provide apparatus and method for the direct electrowinning of metal from previously untreated crushed ore under conditions of continuous high current density to recover as product a metal of dense crystalline form and high purity.

Still another object is the provision of ways and means for electrowinning of metals directly from crushed ore in which a high efiiciency of extraction is achieved without resort to continuous purification and recycle of electrolyte solution.

A related object is the provision of ways and means for achieving the foregoing object at high current efiiciency with resultant low operating costs.

Another important object is the provision of apparatus for achieving the foregoing objects which is low in initial cost, simple and economic in operation and maintenance and which is adaptable to use in a wide variety of physical locations.

A further object is the provision of an electrolytic cell that is adaptable to use either as a single unit or in series operation.

Another object is the provision of a method in which the hydrogen gas released at the cathode is immediately combined with sulfate to form sulfuric acid in the cell that in turn dissolves additional copper from the ore in slurry to render it available for immediate electrodeposition.

Still another object is the provision of apparatus and a method of operating the same in which the cathode is so constructed that it may be removed from the cell as a unit or in sub-elements for service or harvest of the recovered metal.

The present invention overcomes the difficulties of prior electrowinning and achieves the foregoing objects by providing ways and means for direct electrowinning of metals from ores at high capacities through the use of high current densities and which achieve high current efficiency to extract pure dense metal of crystalline form. The invention is predicated upon our discovery that by maintaining a particular relative movement between the electrodes concommittantly with agitation of the slurry a continuously high current density may he maintained with a resultant high recovery rate of hard pure metal. In

this connection, we have discovered that the major objects of our invention are attained in a structure in which the cathode and anode are maintained in spaced-apart relationship in an electrolyte and a continuous relative sliding movement is maintained therebetween while electrical current is passed through the cell. We have also discovered that it is advantageous for the relative movement between anode and cathode to be discontinuous, that is, reciprocating or oscillating movement rather than continuous. For instance, according to the invention a desirable motion is achieved by forming the cathode as an elongated polygon, suspending it concentrically within a larger but similarly shaped anode, then simply oscillating the cathode to and fro about its axis.

According to the invention, crushed ore is slurried with a suitable leach liquid in the cell itself. For instance, copper oxide is mixed with sulfuric acid. Apparently this immediately dissolves some copper to form a copper sulfate solution. As electrolysis proceeds copper deposits on the cathode, some sulfate ions are released and hydrogen gas released at the cathode combines with sulfate to reform H 80 which dissolves additional copper as copper sulfate. Thus, the process is continuous because sulfuric acid is constantly regenerated to work continuously on the copper content of the ore.

In a preferred embodiment of our invention, the cathode is positioned and moved relative to the anode in such a manner that the distance between a given point on the cathode and a given point on the anode is constantly varied while at the same time the slurry is agitated to keep the ore in suspension and scrub the electrodes.

According to the invention this variation of distance between points on the cathode and the anode is achieved by a structure employing a particular construction of the cell tank and cathode which includes means whereby agitation is carried out in a manner that avoids any vortexing, all as will be more fully described hereinafter.

Although the invention will be described with primary reference to the electrowinning of copper, it will be obvious and is intended that it is of application to the recovery of other metals amenable to electrolytic deposition.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood and carried into effect, reference is had to the accompanying drawings and the description thereof which are to be taken as illustrative only and not in limitation of the invention, the scope of which is defined by the appended claims and the equivalents embodied therein.

FIG. 1 is an isometric view of an electrolytic cell embodying the invention.

FIG. 2 is a side sectional view of the cell taken in the plane of line 22 of FIG. 1 as looking in the direction of arrows 2.

FIG. 3 is a partial view of the cell of FIG. 1 taken as looking down in to the cell.

FIG. 4 is a simplified sketch illustrating a drive arrangement for oscillating the cathode.

FIG. 5 is a bottom end view of the shaft illustrating an electrical connection in place on the commutator.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated, the cell comprises an elongated polygonal vessel, in this case an octagonal tank, 11 with an outer wall 12 of plastic or other non-conductive material and lined with a suitable insoluble anode 13 which is connected to a source of direct current (not shown) by a suitable conductor 14. The tank is also provided with a nonconductive inner wall 16, also an elongated polygon, octagonal in shape, and lined with anode material 13 so that the cell defined between the inner and outer walls is an annulus or trough concentric about the tank center. The inner anode is also connected to the direct current source by a conductor 14.

A central shaft 17 extends up through the tank concentrically within the inner wall and is journalled for rotation in suitable bearings 18 adjacent the top and bottom of the inner wall. The bearings 18 are adapted to absorb both radial and thrust load of the shaft. Fixed adjacent the bottom of the central shaft 17 is a lever arm 19 (FIGS. 2 and 4) which, as illustrated in FIG. 4, is in turn connected by a further lever 21 to a pitman wheel 22 driven by a suitable low speed or geared motor 23 to effect oscillation of the shaft 17 upon rotation of the wheel.

A cathode, generally designated 24, is connected to and supported from the top of the central shaft to oscillate therewith. The cathode comprises an elongated cage-like structure including frame members 26 and supports 27 from which the frame members depend. The cage is itself preferably non-conductive and is formed with slots 30 in the upright frame members adapted to snugly receive sheets 28 of metal cathodes. Each of the cathode strips is connected to the negative side of the current source by means of conductors 29 interconnecting cathode and the shaft. The shaft 17 is a conductor and is provided adjacent its lower end with a commutator 31 which connects through a suitable contact or brush device 32 to the negative side of the DC power source. It will be noted that in the illustrated embodiment, the assembled cathode is an elongated structure that, like the cell itself, as shaped as a polygon when viewed from the top.

Feed is introduced into the cell through a suitable conduit 33 and a maximum level in the cell is established by an over-flow outlet 34. A valved conduit 36 is provided for draining or cleanout.

Fins or partial blades 37 are secured to the inside and outside of the cathode to oscillate therewith thereby to agitate the slurry but at the same time avoid any unidirectional liquid flow wihthin the cell. That is, slurry or other liquid is swished to and fro within the cell to achieve a random turbulence but no vortexing flow is established. In this connection, the fiat wall sections of the anode 13 tend to inhibit rotation of the slurry and thus also contribute to turbulent non-vortexing agitation.

To insure circulation of the slurry throughout the cell against the anode 13 at both the outer wall 12 and the inner wall 16 as well as on both sides of the cathode, the cathode is sized to terminate above the cell floor thereby leaving openings under which the slurry may flow. It is also desirable to provide openings adjacent the top of one or more cathode strips as a further aid to circulation.

An alternate construction, not precisely illustrated, contemplates vertical slots in the cathode cage with associated angled buffles to scoop slurry into the cage. A plurality of such openings are provided, some facing in one direction and some in the other so that slurry is scooped into and through the cage in each direction of cage rotation.

The cathode is suspended midway between the inner and outer anodes. As the cathode oscillates it, aided especially by the action of the fins 37, and to a lesser degree by the flat cathodes, effects a wiping action between the slurry and the surfaces of the electrodes. At the same time there is a continuous changing of position between any given point on the cathode and the anode. Illustratively, consider point P on the cathode. The distance L between point P and a similar point P on the anode will constantly change as the cathode oscillates. When the cathode is in the position shown and parallel to its nearest facing anode, the distance L is at a minimum. As the cathode oscillates, distance L continuously increases and decreases. Another way to consider this is to visualize each cathode as comprising an infinite number of vertical lines each of which is successively moved from a maximum distance from the anode gradually toward the anode to a minimum distance thence away again to the maximum spacing.

Maximum current probably flows at the nearest point and vice-versa. Whatever the reason, the polygon shaped cell produced superior results compared to a cylindrical vessel.

It will be noted that any given point P on the oscillating cathode moves in a fixed circular path about the shaft and this achieves a back and forth sliding motion as well as to-and-fro motion relative to the anode. It is not known which, if ether, of these motions is dominant insofar as results are concerned.

In tests, a cell of cylindrical configuration was compared to the octagonal cell illustrated. The octagonal cell demonstrated current efficiencies of about 80% while, under identical conditions, in the cylindrical cell only 50% efiiciency was attainable. In other tests the octagonal shape demonstrated current efiiciencies double those achieved in cylindrical cells. The reason for these differences in performance are not fully understood, but it may be theorized that the continuous changing of the distance L between cathode and anode causes a current ripple that somehow contributes to increased efiiciency.

High current densities, at a minimum of five times those employable in usual solution electrolysis, are employed in our cell and the resuting metal has been pure and of hard crystalline form. Again, while the reasons for this high capacity operation are not fully understood, we have observed that these results have only been achieved with an oscillating cathode.

It is believed that the reciprocating relative motion between the anode and cathode makes a significant contribution to the unexpected results attainable by our invention. It is also believed that the polygonal shape of the cell makes an independent contribution to the improved results. This is borne out by the fact that when the cell incorporates both the oscillating cathode and the polygonal shape, as in the illustrated embodiment, we have been able to reach both high current density and high efiiciency to yield a high rate of high quality hard metal deposition at relatively low power cost. Moreover, the copper was uniform in thickness and texture and was free from dendritic buildup.

A test cell built for work on copper, gold and silver ores was constructed generally as illustrated in the drawings. The cell measured 13" between opposite anode surfaces on the outer walls. Each fiat anode section was 6 /2" wide and 24" high and was of standard antimony-lead composition (8% antimony and 92% lead). The inner anode wall, of the same material, measured 3 /2" across and each plate was 1%" wide by 24" long. The cathode frame was constructed from polypropylene. The cathode sheets were 304 stainless steel. The distance between opposite plates was 9" and each cathode sheet was 2%" wide by 24" long. The cell was designed to operate with about 20 of slurry depth and the cathodes were suspended for immersion only to a depth of 18". Thus the effective cathode area was about five square feet. The slots into which the cathodes fit were made to grip the sheets snugly in order to insulate the sharp edges from the effects of the current and thus avoid local disproportionately high currents.

In batch tests the following results were obtained using the test unit as described above and a full wave rectifier as a source of direct current.

EXAMPLE I A previously untreated sample of copper oxide ore from the Mackey, Idaho area, was used. Except for crushing, the ore was exactly as mined and contained 1.7% copper as CuO A thirty pound sample of ore passing a 48 rneh (Tyler) screen was prepared. An aqueous sulfuric acid electrolyte at pH 1.8 was introduced into a cell and 6.7 grams/liter of a mixture of ferric and ferrous sulfate were added, oscillation was commenced and the ore introduced. Direct current was supplied at 2.9 volts and 120 total amperes. The run was continued for one hour, the

cell emptied, the electrolyte decanted, and the tails and deposited metal assayed. The tails contained only .05 copper and the recovered copper, which was in dense hard crystalline form which stripped readily from the stainless steel cathodes, was 97.6% pure and contained only 003% metallic impurities. The decanted electrolyte contained 1.65 g.p.l. of copper, 5.94 g.p.l. ferrous iron, 2.31 g.p.l. ferric iron and 34.3 g.p.l. H 50 Significantly, the electrolyte contained only 1.65 g.p.l. copper. This is to be contrasted with prior cells which are operable only in the range of 25 to 35 g.p.l. copper.

EXAMPLE II The electrolyte from Example I was reintroduced into the cell, agitation commenced and a second 30 pound batch of the same oxide ore introduced while the same electrical current (2.9 volts at 120 total amperes) was applied. Operations were conducted for one hour. At the end of one hour samples were collected. Assay showed the tails to contain only .03% copper while the deposited copper, again in the form of hard metal strips that peeled readily from the steel cathodes, was 99% pure and contained less than .001% metallic impurities.

In the test cell, total cathode area exposed to current was five square feet hence in both examples the current density, at 120 amperes total current, was 24 amperes/ square foot.

In Example I, a current efiiciency of 99.3% was attained while in Example II the current efiiciency was 71.8%. The reduction in efficiency was probably due to increase in the ferric iron content of the electrolyte.

A significant finding in the foregoing and other actual tests on ore slurries, and even with clean solutions of copper in electrolyte, was the ability of the unit to recover the deposited metal in hard dense form even at high current densities. For instance, in one run in which cement copper at 80% copper content was agitated in aqueous H 80 at 1.8 pH, current densities were maintained at 32. amperes/ft. for a total of eight hours yet the recovered metal was in hard sheets and no undesirable powder or burned copper was formed.

It is important that the cathode be oscillated at a speed a sufficient, when coupled with other agitation means such as the fins 37, to avoid polarization at the electrodes. This will be readily determinable empirically in any given cell. The degree of oscillation is also selected to optimize agitation and to insure continuous change in the effective distances between the anode and cathode. The speed and degree of oscillation will also depend uponthe physical dimension of the cell and, in particular, the size and weight of the oscillating cathode. Again, these are features that will vary from case-to-case. Spacing between the cathode and anode should be as close as possible, say, three inches or so, to maximize current flow but at the same time avoid collision between the anode and metal collected on the cathode. The dimensions of the cell and the degree of reciprocation will establish or predetermine the variations in length of the distance L between a given point on the cathode and a given point on the anode. Although the actual amount of variations in distance L in the test cell are small, they are large percentagewise and may change as much as 25%.

-A significant advantage to our new cell is the facility with which the cathode cage may be removed and a fresh one substituted or only single cathode strips removed. Also, if desirable only part of the cathode strips need be inserted.

The exact shape of the cell wall and cathode cage are not fixed. That is, the anode need not necessarily be an octagon, but may be any other regular polygon such as a hexagon. Although best results are achieved with a polygonal cathode in smaller cells, the cathode may approach cylindrical yet still achieve the variations in the distance L between the electrodes upon oscillation.

The necessary random turbulent agitation may, in special structures, be achieved by separate impellers immersed in the cell rather than by fins or other means on the cathode. The main goal in agitation is that the entire slurry be subjected to continuous turbulent agitation to minimize or scrub away polarizaion effects at the electrodes while maximizing mixing of released hydrogen with the liquid to insure acid regeneration as well as through mixing of acid with ore thereby to insure continuous presentation of metallic ions at the cathode and at the same time avoiding any set flow pattern such as vortexing within the cell.

The invention has been described with particular reference to a single octagonal cell used for electrowinning of copper directly from crushed ores. It will be appreciated that the apparatus and method of the invention can be, and indeed have been, employed to extract silver and gold and other metals from ores. Also the invention has application in electrorefining. In commercial applications it is anticipated that a plurality of cells will be arranged in series with concurrent flow of new ore and strong recycled electrolyte therethrough. It should be noted, however, that even with a single cell continuous, as opposed to batch, operation may be achieved by feeding new slurry into the cell while a proportionate amount of slurry is overflowed.

Although probably not as convenient, it is likely that under special circumstances the anodes and cathodes may be reversed in our cell. That is, the oscillating cathode 24 can be connected to the positive pole to thus serve as the anode while the conductors 13 are connected to the negative pole and thus serve as cathodes upon which the metal deposits. In such a structure, the same relative motions between the electrodes will be achieved, but the poles will be reversed. Obviously, all the usual electrode materials may be employed.

What is claimed is:

1. An electrolytic cell comprising a tank having a bottom and an upstanding marginal side wall said wall at least on its inner surface being adapted to conduct electrical current thereby the form an anode, a shaft extending upwardly through the floor of said tank and journalled for rotational movement with respect to said tank, a cathode formed as an elongated cage and secured to said shaft to be concentric thereabout and extend downwardly into said tank parallel to the tank walls and concentrically spaced from said anode, means including an inlet and outlet for introducing and maintaining a mass of material to be treated in said tank, agitation means in said tank enabling continuous random agitation of material therein while inhibiting creation of fixed flow paths and vortexing of said material, means connecting said anode and said cathode to positive and negative terminals of a direct current power source, and drive means operable on said shaft to rotate the same back and forth thereby to effect oscillation of said shaft and said cathode within said tank.

2. An electrolytic cell according to preceding claim 1 in which the anode material on the tank wall is arranged to define a regular polygon.

3. An electrolytic cell according to preceding claim 1 in which said cathode is formed as a polygon and said agitating means includes fins secured to said cathode for oscillation therewith.

4. An electrolytic cell according to preceding claim 1 in which said cathode comprises a frame secured to and movable with said shaft a plurality of metal plates and means removably mounting said plates in said frame.

5. An electrolytic cell according to preceding claim 4 in which said means removably mounting said plates in said frame include pieces of non-conductive material having slots into which the edges of said plates are received.

'6. An electrolytic cell according to preceding claim 1 with the addition of an inner wall in said tank located concentrically about said shaft and between said shaft and said cathode to define with said outer wall an endless trough to contain material to be treated and said cathode is positioned to depend into said trough for oscillation therein, and at least the surface of said inner wall facing said outer wall is adapted to conduct electricity and is connected to the positive terminal of said power source thereby to act as an anode.

7. An electrolytic cell according to preceding claim 6 in which the conductive surfaces of both the inner and outer walls form regular polygons of more than four sides.

8. An electrolytic cell according to preceding claim 7 in which vertical fins are provided on said cathode to extend therefrom radially outwardly toward said outer wall and radially inwardly toward said inner wall.

9. An electrolytic cell comprising a tank having a bottom and an upstanding marginal side wall said wall at least on its inner surface being electrically conductive to form a first electrode, a shaft extending upwardly through the floor of said tank and journalled for rotational movement with respect to said tank wall, a structure including electrically conductive elements forming a second electrode constructed as an elongated cage, said cage being secured to said shaft to be concentric thereabout and extend downwardly into said tank parallel to and concentrically spaced from said tank wall forming said first electrode, means including an inlet and outlet for introducing and maintaining a mass of material to be treated in said tank, agitation means in said tank for effecting continuous random turbulent agitation of material therein While inhibiting creation of fixed flow paths and vortexing of said material, means connecting said first electrode to one pole of a source of direct current and said second electrode to the opposite pole of said source, and drive means operable on said shaft to rotate the same back and forth thereby to effect oscillation of said shaft and said second electrode within said tank.

10. In the method for recovering metal from a slurry of finely divided metal bearing solids in an electrolyte in which an elongated anode and an elongated cathode are suspended vertically with electrical current flowing therebetween, the improvement which comprises the steps of inducing random turbulent agitation of said electrolyte and metal bearing materials to inhibit polarization while simultaneously effecting reciprocating relative motion between the cathode and anode to effect continuous variation within preselected limits in the distance between a given point on the cathode and a given point on the anode by oscillating said cathode about its vertical axis.

References Cited UNITED STATES PATENTS 3,507,770 4/1970 Fleming 204-106 2,028,285 1/1936 Jephson et a1. 204-212 791,341 5/1905 Harrison 204-237 545,328 8/1895 Wiggin 204-222 THOMAS M. TUFARIELLO, Primary Examiner U.S. Cl. X.R. 

